From: Shakeel Butt <shakeelb@google.com>
To: Roman Gushchin <guro@fb.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>,
Linux MM <linux-mm@kvack.org>,
Andrew Morton <akpm@linux-foundation.org>,
Michal Hocko <mhocko@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Kernel Team <Kernel-team@fb.com>,
"stable@vger.kernel.org" <stable@vger.kernel.org>,
Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm()
Date: Wed, 6 Nov 2019 18:21:48 -0800 [thread overview]
Message-ID: <CALvZod43v4Xx6YzhN8ku3=YrPVGJoK-8mUejg1f29a1jxL1-ug@mail.gmail.com> (raw)
In-Reply-To: <20191107014307.GA1158@castle.dhcp.thefacebook.com>
On Wed, Nov 6, 2019 at 5:43 PM Roman Gushchin <guro@fb.com> wrote:
>
> On Wed, Nov 06, 2019 at 05:25:26PM -0800, Shakeel Butt wrote:
> > On Wed, Nov 6, 2019 at 4:22 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
> > >
> > > On Wed, Nov 06, 2019 at 02:51:30PM -0800, Roman Gushchin wrote:
> > > > We've encountered a rcu stall in get_mem_cgroup_from_mm():
> > > >
> > > > rcu: INFO: rcu_sched self-detected stall on CPU
> > > > rcu: 33-....: (21000 ticks this GP) idle=6c6/1/0x4000000000000002 softirq=35441/35441 fqs=5017
> > > > (t=21031 jiffies g=324821 q=95837) NMI backtrace for cpu 33
> > > > <...>
> > > > RIP: 0010:get_mem_cgroup_from_mm+0x2f/0x90
> > > > <...>
> > > > __memcg_kmem_charge+0x55/0x140
> > > > __alloc_pages_nodemask+0x267/0x320
> > > > pipe_write+0x1ad/0x400
> > > > new_sync_write+0x127/0x1c0
> > > > __kernel_write+0x4f/0xf0
> > > > dump_emit+0x91/0xc0
> > > > writenote+0xa0/0xc0
> > > > elf_core_dump+0x11af/0x1430
> > > > do_coredump+0xc65/0xee0
> > > > ? unix_stream_sendmsg+0x37d/0x3b0
> > > > get_signal+0x132/0x7c0
> > > > do_signal+0x36/0x640
> > > > ? recalc_sigpending+0x17/0x50
> > > > exit_to_usermode_loop+0x61/0xd0
> > > > do_syscall_64+0xd4/0x100
> > > > entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > > >
> > > > The problem is caused by an exiting task which is associated with
> > > > an offline memcg. We're iterating over and over in the
> > > > do {} while (!css_tryget_online()) loop, but obviously the memcg won't
> > > > become online and the exiting task won't be migrated to a live memcg.
> > > >
> > > > Let's fix it by switching from css_tryget_online() to css_tryget().
> > > >
> > > > As css_tryget_online() cannot guarantee that the memcg won't go
> > > > offline, the check is usually useless, except some rare cases
> > > > when for example it determines if something should be presented
> > > > to a user.
> > > >
> > > > A similar problem is described by commit 18fa84a2db0e ("cgroup: Use
> > > > css_tryget() instead of css_tryget_online() in task_get_css()").
> > > >
> > > > Signed-off-by: Roman Gushchin <guro@fb.com>
> > > > Cc: stable@vger.kernel.org
> > > > Cc: Tejun Heo <tj@kernel.org>
> > >
> > > Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> > >
> > > The bug aside, it doesn't matter whether the cgroup is online for the
> > > callers. It used to matter when offlining needed to evacuate all
> > > charges from the memcg, and so needed to prevent new ones from showing
> > > up, but we don't care now.
> >
> > Should get_mem_cgroup_from_current() and get_mem_cgroup_from_page() be
> > switched to css_tryget() as well then?
>
> In those case it can't cause a rcu stall, so it's not a so urgent.
> But you are right, we should probably do the same here. I'll look
> at all remaining callers and prepare the patchset.
>
> I'll also probably rename it to css_tryget_if_online() to make obvious
> that it doesn't hold the cgroup from being onlined.
>
SGTM
next prev parent reply other threads:[~2019-11-07 2:22 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-06 22:51 [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm() Roman Gushchin
2019-11-06 22:51 ` [PATCH 2/2] mm: hugetlb: switch to css_tryget() in hugetlb_cgroup_charge_cgroup() Roman Gushchin
2019-11-07 0:26 ` Johannes Weiner
2019-11-07 2:31 ` Shakeel Butt
2019-11-07 15:44 ` Tejun Heo
2019-11-07 0:22 ` [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm() Johannes Weiner
2019-11-07 1:25 ` Shakeel Butt
2019-11-07 1:43 ` Johannes Weiner
2019-11-07 1:43 ` Roman Gushchin
2019-11-07 2:21 ` Shakeel Butt [this message]
2019-11-07 2:28 ` Shakeel Butt
2019-11-07 12:21 ` Michal Hocko
2019-11-07 15:43 ` Tejun Heo
2019-11-07 16:42 ` Roman Gushchin
2019-11-07 17:02 ` Michal Hocko
2019-11-07 22:41 ` Roman Gushchin
2019-11-08 8:53 ` Michal Hocko
2019-11-07 15:43 ` Tejun Heo
2019-11-13 16:29 ` Michal Koutný
2019-11-13 17:08 ` Roman Gushchin
2019-11-14 19:16 ` Michal Hocko
2019-11-14 19:20 ` Tejun Heo
2019-11-14 19:33 ` Michal Hocko
2019-11-14 19:37 ` Tejun Heo
2019-11-15 17:40 ` Michal Hocko
2019-11-15 17:45 ` Tejun Heo
2019-11-15 17:47 ` Michal Hocko
2019-11-15 17:48 ` Tejun Heo
2019-11-15 18:03 ` Michal Hocko
2019-11-15 18:07 ` Roman Gushchin
2019-11-18 9:43 ` Michal Hocko
2019-11-15 18:13 ` Roman Gushchin
2019-11-21 15:28 ` Michal Koutný
2019-11-22 8:20 ` Michal Hocko
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